fix: reduce AGC aggressiveness to prevent noise amplification

Lower maxGain from 8x to 4x (12dB) and target level from 18% to 12%.
The previous 8x gain would amplify quiet noise floors by 18dB,
transforming room tone and electrical hum into audible static. The
compression threshold is raised from 70% to 85% with a gentler 2:1
ratio instead of 3:1, reducing pumping distortion. The soft limiter
threshold is relaxed from 0.90 to 0.95 with a 0.2 knee instead of
0.1, providing more headroom before limiting engages.

These changes reduce the distortion and unnatural 'processed' sound
that other users would hear from barnard clients.

audio/agc.go

@@ -21,16 +21,16 @@ type AGC struct {
 // NewAGC creates a new AGC processor with sensible defaults for voice
 func NewAGC() *AGC {
 	return &AGC{
-		targetLevel:    0.18,  // Target 18% of max amplitude (balanced level)
-		maxGain:        8.0,   // Maximum 8x gain (about 18dB)
-		minGain:        0.1,   // Minimum 0.1x gain (-20dB)
-		attackTime:     0.005, // Fast attack (5ms)
-		releaseTime:    0.1,   // Slower release (100ms)
+		targetLevel:    0.12,  // Target 12% of max amplitude (conservative level)
+		maxGain:        4.0,   // Maximum 4x gain (about 12dB)
+		minGain:        0.25,  // Minimum 0.25x gain (-12dB)
+		attackTime:     0.008, // Fast attack (8ms)
+		releaseTime:    0.15,  // Slower release (150ms)
 		currentGain:    1.0,   // Start with unity gain
 		envelope:       0.0,   // Start with zero envelope
 		enabled:        true,  // Enable by default
-		compThreshold:  0.7,   // Compress signals above 70%
-		compRatio:      3.0,   // 3:1 compression ratio
+		compThreshold:  0.85,  // Compress signals above 85%
+		compRatio:      2.0,   // 2:1 compression ratio (gentler)
 	}
 }
 
@@ -106,10 +106,10 @@ func (agc *AGC) ProcessSamples(samples []int16) {
 		}
 
 		// Soft limiting to prevent clipping
-		if processed > 0.90 {
-			processed = 0.90 + (processed-0.90)*0.1
-		} else if processed < -0.90 {
-			processed = -0.90 + (processed+0.90)*0.1
+		if processed > 0.95 {
+			processed = 0.95 + (processed-0.95)*0.2
+		} else if processed < -0.95 {
+			processed = -0.95 + (processed+0.95)*0.2
 		}
 
 		// Convert back to int16